The goal of this Program Project is to understand the immunopathology of dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS), the severe complications of dengue virus infections. The frequency of DHF/DSS is much greater during secondary infections with dengue virus strains of a different dengue serotype than that which caused the primary infection. These immunopathological results have slowed down vaccine development because of safety concerns. The morbidity and mortality associated with severe dengue infections is a major public health problem which is increasing in frequency. We propose to define the molecular and cellular basis of the immunopathology associated with severe dengue infections. Project #1 consists of clinical investigations in Thailand. Quantitative markers of dengue infection and immune responses that may correlate with progression from uncomplicated dengue fever (D) to DDS/DSS will be determined early in the disease process: the number of dengue-infected peripheral blood mononuclear cells, the levels of cytokines produced as a result of dengue infection of monocytes and activation of T lymphocytes and monocytes. Project #2 will establish a molecular understanding of dengue specific human T lymphocyte and monocyte responses during severe dengue infections. Dengue specific human CD4+ T helper 1 and 2 subsets will be defined at the clonal level. TcR usage in DHF/DSS and D will be defined. Cytokine production from monocytes obtained during acute dengue infections will be determined. Titers in plasma of virus and infectious virus-antibody complexes will be determined. We will learn whether correlations exist between these parameters and disease activity. Project #3 will define the sequence of dengue virus isolates from children with DHF/DSS or D to determine whether there are consistent sequence differences. A Clinical Research Core and a Molecular Immunology Core are needed to support these research projects. An Administrative Core will be responsible for directing, coordinating and overseeing this Program Project. Our ultimate goal is to develop an improved molecular and cellular understanding of the immunopathology DHF/DSS as a basis for the earlier diagnosis of children at highest risk, as a basis for developing logical and safe interventions and vaccine approaches to prevent this severe illness.